In reproducing a picture in natural color of intermediate tones with a conventional color copy machine, color facsimile or printer, etc. it is necessary to have a processing called "color masking process" for correcting the color turbidity of reproduced color attributable to the recording system. Major reasons for the need of color correction are the facts that dye stuffs of three primary colors (3-color) in printing ink have a spectrographic property including unwanted absorption called "subabsorption", and that mixing of 3-color inks causes additivity failure and proportionality failure among the 3-color components.
The color masking process intended to perform high-fidelity color reproduction computes the following masking equation (1) or (2) for a set of 3-color signals of yellow (Y), magenta (M) and cyan (C) or a set of signals of red (R), green (G) and blue (B), and supplies the color correction signals (C', M', Y') or (R', G', B') to the recording system so that the color turbidity is eliminated. EQU (C', M', Y')=f(C, M, Y) (1)
or EQU (R', G', B')=g(R, G, B) (2)
Since equations (1) and (2) generally include nonlinear terms, the following second-order masking is known as a practical method for high-fidelity color reproduction.
A method conceivable to implement such computation with an apparatus is such that color correction results (C', M', Y') for all combinations of inputs (C, M, Y) in the following equation (3) are calculated out in advance, stored as a reference table in the memory and read out by using inputs (C, M, Y). This memory reference system is described in Japanese Patent Unexamined Publication (Kokai) No. 49-106714, for example. The method is flexible in the capability of dealing with various nonlinear functions besides the following equation (3). ##EQU1## where i=1 to 3, j=1 to 9, and (aij) represents a 3-by-9 color correction coefficient matrix.
The memory reference system, however, has a drawback of requiring the storage of the whole results of (C', M', Y') for all combinations of inputs (C, M, Y), and therefore it necessitates a memory of large capacity. Generally, dealing with a color picture of intermediate tones requires 6-bit data (i.e., 2.sup.6 =64 levels) for each of inputs (C, M, Y). There are combinations of 2.sup.6+6+6 =2.sup.18 in number for obtaining each of C', M' and Y', requiring for each color memory devices 301-303 of 2.sup.18 .times.6 bits (about 1.57.times.106 bits) and such an apparatus is expensive to build. The same problem as mentioned above arises for the case of 3-signal set (R, G, B).